The present invention relates to a biomagnetic field measuring apparatus using SQUID (Superconducting Quantum Interference Device) sensors, which are superconducting devices for detecting a very weak magnetic field generated from a living body. Particularly, the invention is concerned with a biomagnetic field measuring apparatus and method capable of easily obtaining a combined image of functional information on the activity of the heart of a subject to be inspected and a morphological image of the heart, as well as a data processing method and a positioning method for the subject to be inspected using the biomagnetic field measuring apparatus.
In a conventional biomagnetic field measuring apparatus for measuring the function of the brain, as shown in FIG. 16, plural detecting coils 12 are arranged on a bottom 11 of Dewar whose external form is in conformity with the curvature of the head, magnetic field generating coils 13, which are mounted at plural positions of the head, are energized and the resulting magnetic field is detected by the detecting coils 12. Further, a relation between the magnetic field generated by the magnetic field generating coils 13 and the output of the detecting coils 12 is simulated. The positions of the magnetic field generating coils 13, which minimize the difference between the measured data detected by the detecting coils 12 and the output of the detecting coils 12 after the simulation, are estimated to specify position coordinates at the head positions where the magnetic field generating coils 13 are arranged (see, for example, JP-A-No. Hei 4-303416).
As shown in FIG. 17, in measuring a morphological image of a head by an MRI (magnetic resonance imaging) device, MRI markers 21 are arranged at the same positions as the head positions where the magnetic generating coils 13 shown in FIG. 16 are arranged, and a tomogram of the entire head, including the MRI markers 21, is measured. Then, position coordinates of the MRI markers 21 are specified using an MRI image (see, for example, JP-A-No. Hei 4-303416).
In combining the results of measurement of the brain magnetic field with the MRI image of the head, which represents a form, there is determined a relation between the position coordinates of the magnetic field generating coils 13 and the position coordinates of the MRI markers 21. For example, when the position of an active brain site obtained by measuring the brain magnetic field is to be displayed on the morphological image, a tomogram of the brain is reconstructed so as to include coordinates corresponding to the position of the active site with use of the head tomogram obtained by the MRI device, and then the active site of the brain and the MRI image are combined and displayed (see, for example, A. Uchida et al., AVSth based Brain Activity Analysis System with a Real Head Shape, Recent Advances in Biomagnetism, Edited by Y. Yoshimoto et al., Tohoku University Press, pp.177-180, 1999).
In connection with a biomagnetic field measuring apparatus, various methods have been reported for establishing a positional relation between a subject to be inspected on a bed and a Dewar (see, for example, JP-A-Nos. Hei 3-244433, Hei 2-180244. and Hei 4-109929).
In case of applying the above conventional technique in the measurement of a brain magnetic field to the measurement of a biomagnetic field generated from the chest of a living body, the conventional technique involves a problem in that a complicated simulation calculation is needed for specifying position coordinates of the magnetic field generating coils arranged to specify position coordinates of a head in the measurement of a brain magnetic field and a problem in that, in the case of an MRI image, it is necessary to read MRI markers.
Moreover, in combining the results of having measured a brain magnetic field with an MRI image, it is necessary to determine a relation between coordinates of the magnetic field generating coils and coordinates of MRI markers, and it is also necessary to perform a calculation for reconstructing a tomogram of the head so as to include coordinates corresponding to the position of an active site of the brain with use of a tomogram obtained by an MRI device.
It is an object of the present invention to provide a biomagnetic field measuring apparatus which is capable of solving the above-mentioned problems of the prior art. Particularly, the present invention aims at providing a biomagnetic field measuring apparatus and a method capable of realizing, in a short time and easily, an operation for aligning the position of the heart of a subject to be inspected with a sensor array and an operation for obtaining a large signal output from SQUID (Superconducting Quantum Interference Device) sensors.
It is another object of the present invention to provide a biomagnetic field measuring apparatus which is capable of easily forming and displaying a combined image of functional information on the activity of the heart obtained from the biomagnetic field measuring apparatus with a morphological image obtained by an image pick-up device other than the biomagnetic field measuring apparatus.
It is a further object of the present invention to provide a data processing method for displaying a combined image in the biomagnetic field measuring apparatus, as well as a positioning method which is suitable for establishing a position of a subject to be inspected using the biomagnetic field measuring apparatus at the time of displaying the combined image.
The following description is directed to typical constructions of biomagnetic field measuring apparatuses according to the present invention.
A biomagnetic field measuring apparatus according to the present invention is provided with a bed for supporting a subject to be inspected thereon, a bed support, a cryostat for cooling a plurality of SQUID sensors, and a gantry fixed to a floor surface for holding the cryostat at a known distance with respect to the floor surface. The bottom of the cryostat and an upper surface of the bed are positioned substantially in parallel with the floor surface.
The cryostat is provided at an outer peripheral surface of its bottom with an xz marking which represents an xz plane of a coordinate system (x, y, z) and a yz marking which represents a yz plane of the coordinate system. In the coordinate system (x, y, z), the xy plane is parallel to the bottom of the cryostat and z axis is perpendicular to the bottom of the cryostat.
A plurality of SQUID sensors are arranged, respectively, in x and y directions near an inner bottom of the cryostat to detect a component in the z direction of a magnetic field which is generated, for example, from the heart of the subject to be inspected. As such plural SQUID sensors, fluxmeters which detect components in both x and y directions of a magnetic field generated from the heart of the subject to be inspected may be used.
An optical system is used to adjust a positional relation between the bottom of the cryostat and the bed. The optical system comprises a first laser source for generating a first sectorial laser beam which spreads sectorially in the xz plane, a second laser source for generating a second sectorial laser beam which spreads sectorially in yz plane, and a third laser source for generating a dot-like laser beam which is radiated to the bed surface obliquely across the first and second sectorial laser beams. The first laser source is fixed to a frame which is secured to the gantry; the second laser source is fixed to a frame which is secured to the bed support; and the third laser source is fixed to a frame which is secured to any of the floor surface, ceiling, and wall surface.
As means for changing irradiating directions of the laser beams generated respectively from the three laser sources, there are first position changing means which changes the irradiating direction of the first sectorial laser beam so as to irradiate the xz marking, second position changing means which changes the irradiating direction of the second sectorial laser beam so as to irradiate the yz marking, and third position changing means which changes the irradiating direction of the dot-like laser beam so as to irradiate a crossing line between the first and second sectorial laser beams and also irradiate a crossing point between the z axis and the bed surface.
As means for moving the bed position with respect to the bottom of the cryostat, there are x direction moving means which moves the bed support in x direction on the floor surface, y direction moving means which moves the bed in y direction on the bed support, and z direction moving means which moves the bed in z direction on the bed support.
With movement of the bed position relative to the cryostat bottom, the distance between the bed and the floor surface is measured automatically by distance measuring means and the measured value is displayed on a display unit.
According to this construction, the first and second sectorial laser beams from the first and second laser sources, respectively, and the dot-like laser beam from the third laser source have their irradiating directions changed and the bed is moved in x, y, and z directions. With a simple construction, a vertical position of the bed can be measured and it is possible to adjust the positional relation between the subject to be inspected on the bed and the cryostat bottom.
In another typical construction of a biomagnetic field measuring apparatus according to the present invention, a plurality of SQUID sensors for detecting a normal line component of a magnetic field generated from the heart of a subject to be inspected are arranged in two dimensions on an inner bottom of a cryostat (Dewar) and are cooled to a low temperature. The SQUID sensors are driven by a drive circuit and magnetic waveform signals of the normal line component detected by the SQUID sensors are collected by means of a processing unit, such as a computer, which performs an arithmetic processing and controls various portions of the apparatus. Prior to the measurement, a first marker indicative of a first reference point is disposed on the body surface at a first point of the chest of the subject to be inspected and a second marker indicative of a second reference point is disposed on the body surface at a second point of the chest.
A coordinate system (x, y, z) is established in the biomagnetic field measuring apparatus and a positional relation between the chest surface of the subject to be inspected on the bed and a bottom surface of the Dewar is adjusted using a total of three laser beams, which include a sectorial laser beam spreading sectorially in the xz plane, a sectorial laser beam spreading sectorially in a plane parallel to the yz plane, and a dot-like laser beam which is radiated to the bed surface obliquely across those two sectorial laser beams. The xy plane of the coordinate system (x, y, z) is set on a measuring plane for measurement with the SQUID sensors. The bottom of Dewar is parallel to all of the xy plane, measuring plane and bed upper surface, and the distance between the bed upper surface and the Dewar bottom is known.
The bed is moved in the z direction up to a position sufficiently higher than the height of the body surface of the subject to be inspected, which height has been measured with the bed adjusted to its lowest height. Then using the three laser beams, the irradiating direction of the dot-like laser beam is set so as to irradiate a crossing point between the z axis and the bed surface, and the distance between the Dewar bottom and the bed upper surface is measured. The bed is brought down to a low position, and the subject to be inspected is laid on the bed. Then the sectorial laser spread in the yz plane is moved in the x direction so as to pass through the first and second reference points, and the position of the subject to be inspected is adjusted so that the line joining the first and second reference points becomes parallel or coincident with one direction in which the centers of the SQUID sensors are arranged.
Next, the bed is moved in the y direction so that the sectorial laser spread in the xz plane passes through the first reference point. Further, the bed is moved in the x direction so that the sectorial laser beam spread in a plane parallel to the yz plane becomes coincident with the yz plane, and, thereafter, the bed is moved in the z direction until the irradiation point of the dot-like laser beam becomes coincident with the first reference point. Then, the bed is moved in the z direction until the body surface of the subject to be inspected comes into contact with the bottom of the Dewar and the amount of the movement is detected.
Since an initial distance between the bottom of the Dewar and the first reference point and the amount of the movement of the bed are known, it is possible to determine the distance between the Dewar bottom and the first reference point when the body surface contacts the bottom of the Dewar. Since the positions of processus xiphoideus and incisura jugularis can be determined by touch easily with a high reproducibility, it is preferable to select, as the first point, a body surface position of processus xiphoideus of the subject to be inspected and, as the second point, to select a body surface position of incisura jugularis of the subject to be inspected.
The processing unit executes a data processing method comprising (1) a processing of forming, from a magnetic waveform signal, an image which represents functional information relating to the activity of the heart of the subject to be inspected; (2) a processing in which the pixel size of the image representing the functional information is made coincident with the pixel size of a morphological image of the chest of the subject to be inspected, and there is formed a functional image having the same pixel size as that of the morphological image in which a first marker indicating the first reference point is disposed on the body surface at the first point of the chest of the subject to be inspected; (3) a processing of bringing the position of the first reference point in the functional image into coincidence with the position of the first marker in the morphological image; and, (4) a processing of forming a combined image of the functional image and the morphological image. Prior to the processing (4) there is performed a processing (3xe2x80x2) of rotating the morphological image around the first reference point and thereby making the pixel arrangement direction in the body axis direction of the subject to be inspected in the morphological image and that in the functional image coincident with each other.
Further, in connection with the processing (1), the processing unit executes the following data processing method comprising steps (a) to (e). (a) There are performed a processing of estimating an activated position of the heart of the subject to be inspected as a current source, using a magnetic waveform signal of a magnetic field component in a normal line direction which has been measured, and a processing of forming an image including the position of the current source as an image which represents functional information. Further, a processing is carried out to obtain a tangential magnetic field component of a magnetic field generated from the heart of the subject to be inspected, and the following processings are conducted using a magnetic waveform signal of the tangential magnetic field component. (b) There is performed a processing of forming an isomagnetic field map wherein coordinate points equal in magnetic field intensity are connected together and which is obtained as an image representing functional information. (c) There is performed a processing of forming an arrow map which represents an activated position of the heart of the subject to be inspected as a two-dimensional current distribution and which is obtained as an image representing functional information. (d) There is performed a processing of integrating a magnetic waveform in a temporal interval which contains a specific period of the heart activity of the subject to be inspected, to determine an integral intensity, and forming an isointegral map wherein coordinate points equal in integral intensity are connected together and which is obtained as an image representing functional information. (e) There is performed a processing of integrating magnetic waveforms of tangential magnetic field components in a temporal interval including two different periods of the heart activity of the subject to be inspected, to determine integral intensities, and forming an isointegral map wherein coordinate points equal in the value of difference between integral intensities in the temporal period including two different periods are connected together and which is obtained as an image representing functional information.
The morphological image is selected, for example, from any of a tomogram nearly parallel or perpendicular to the chest surface of the subject to be inspected, which has been photographed by an MRI device, a tomogram nearly parallel or perpendicular to the chest surface of the subject to be inspected, which has been photographed by a three-dimensional XCT (X-ray computed tomography) device, and an X-ray image of the subject to be inspected, which has been photographed by an X-ray camera. Using the thus-selected image and any of the images representing functional information which have been obtained in the above processings (a) to (e), the processing unit executes the data processing method including the foregoing processings (2) to (4) and (3xe2x80x2).
According to the constitution of the present invention described above, prior to detecting a magnetic field generated from the heart of the subject to be inspected, the positional relation between the chest surface of the subject to be inspected on the bed and the bottom surface of the Dewar can be adjusted with a simple construction using a total of three laser beams, which include two sectorial laser beams and one dot-like laser beam.
As a result, substantially the whole of projection of the heart on the sensor array surface is positioned within the region of the sensor array, and the body surface of the chest of the subject to be inspected comes into contact with the lower surface of the Dewar, whereby there is obtained a large signal output. The above positional relation adjusting operation can be done in a short time and easily.
According to the present invention, moreover, such complicated calculations as simulation calculation relating to the generation of a magnetic field for estimating a magnetic field source and a reconstructing calculation for a tomogram are not performed, and a combined image can be formed and displayed easily from biofunctional information and a morphological image (tomogram) or a transmitted image, such as an X-ray image of the chest obtained by an X-ray camera. The biofunctional information is obtained by the biomagnetic field measuring apparatus, and is especially functional information on the activity of the heart, which is represented in terms of an isomagnetic field map, an arrow map, or an isointegral map, obtained from a magnetic waveform resulting from measuring a magnetic field generated from the heart, or the result of having estimated the position of a current dipole, the morphological image being obtained by a magnetic resonance imaging (MRI) device or a three-dimensional X-ray computed tomography (XCT) device and being substantially parallel or perpendicular to the chest surface.
In the MRI device or the three-dimensional XCT device, in many cases, the bed for supporting the subject to be inspected thereon is held horizontally, and, in image photographing, the subject to be inspected is laid on the bed so that the long axis direction of the bed and the body axis of the subject to be inspected are almost coincident with each other. In obtaining a chest X-ray image (X-ray transmitted image) using an X-ray camera, the subject to be inspected is in many cases photographed in a standing state or in an upright sitting state on a chair or is sometimes photographed while lying on a bed in a general hospital building.
Even when the long axis direction of the bed and the body axis of the subject to be inspected are not in exact agreement with each other during photographing in the MRI device or three-dimensional XCT device, the body axis direction of the subject to be inspected in the morphological image and the pixel arrangement direction in the body axis direction of the subject to be inspected (the direction joining the first and second reference points) in the functional image can be made coincident with each other by performing the foregoing processing (3xe2x80x2).
More particularly, since it is possible to prepare a combined image of the functional image and an image obtained by rotating the morphological image around the first reference point (a central position of an MRI marker image or of an X-ray marker image), it is possible to obtain a more accurate combined image of both the functional image and the morphological image. For example, such an accurate combined image can be obtained by rotating the morphological image around the first reference point (a central position of an X-ray marker image) so that a center line of the backbone in a chest X-ray image (X-ray transmitted image) and the pixel arrangement direction in the body axis direction of the subject to be inspected in the functional image become coincident with each other.
A typical construction of the biomagnetic field measuring apparatus for obtaining a combined image of an image which represents biofunctional information with a morphological image will now be outlined with reference to FIG. 2. A marker 37 which represents a first reference point is disposed on the surface of processus xiphoideus of a subject 35 to be inspected, and a second marker 38 which represents a second reference point is disposed on the surface of incisura jugularis. The chest of the subject to be inspected is disposed below the bottom of a cryostat 36 in such a manner that a line joining the first and second reference points extends along one direction of the arrangement of SQUID sensors in the interior of the cryostat.
A processing unit executes (1) a processing of forming, from a magnetic waveform signal, an image which represents functional information relating to the activity of the heart of a subject to be inspected; (2) a processing of making the pixel size of a morphological image, including the heart photographed by an image pickup device, coincident with the pixel size of an image which represents functional information to form a functional image having the same pixel size as that of the morphological image, with a first-marker indicative of a first reference point being disposed on the surface of processus xiphoideus; (3) a processing of making the position of the first reference point in the functional image and that in the morphological image coincident with each other; and (4) preparing a combined image of both the functional image and the morphological image. In this way the combined image can be obtained easily without requiring any complicated calculation.
According to this construction, particularly in detecting a magnetic field generated from the heart of the subject to be inspected, substantially the whole of the projection of the heart on the sensor array surface is positioned within the region of the sensor array, and the chest surface of the subject to be inspected comes into contact with a lower surface of the Dewar, whereby the operation for affording a large signal output can be effected in a short time and easily.
According to this construction, moreover, without performing such complicated calculations as a simulation calculation relating to the generation of a magnetic field and a tomogram reconstructing calculation, it is possible to easily form and display a combined image of functional information and a morphological image (tomogram) or a transmitted image, such as a chest X-ray image obtained by an X-ray camera, the functional information relating to the activity of the heart and being represented in terms of an isomagnetic field map, an arrow map, or an isointegral map, obtained from a magnetic waveform resulting from measuring a magnetic field generated from the heart, or the result of having estimated the position of a current dipole, the morphological image being obtained by an MRI device or a three-dimensional XCT device and being substantially parallel to the chest surface.